Gaseous fuel injectors are known to use solenoid actuators to move a plunger or disc style armature to open an injection valve. The armature has a rubber seal (also known as a shutter) that dynamically seals around a valve seat when the injection valve is closed. These types of gaseous fuel injectors have very low leakage and wear, allowing for a very long service life, and are relatively inexpensive to produce. To balance part-to-part injector performance, the stroke of the injector is normally limited to a lower value than that which gives maximum mass flow so that the injectors can be balanced by adjusting the exact stroke on the production line. The injector is flow-limited in an area under the armature when the ratio between armature lift (stroke length) and valve orifice area is relatively small. As these injectors age, or even when relatively new, mechanical, chemical and electromagnetic differences will affect relative static and dynamic behavior of the armature motion and injection valve performance. Electromagnetic differences can result from a variety of reasons, including dimensional differences in the injection valve components, air-gaps, coil windings, seal volume, wire harness resistance, chemical swelling of elastomers and pin electrical resistance contact variances. The differences in injector performance has been observed, both on test rigs and with parts returned from the field for servicing, to cause large fuel delivery variations, particularly between injectors. Often these affects are very noticeable at low pulse width conditions where the linearity of injection performance is reduced as a result of the plunger bouncing when the injection valve is opened. Also, during cold starting, trace oil, water and wear particles that accumulate in between moving parts (such as between the plunger and the injector body or tube, between the armature seal and the valve seat, and the return spring) may cause the injectors to respond in a “sluggish manner” or not at all. This is due to increased viscous drag, surface tension or even solidification (amorphous, crystalline) of these “contaminants” that are normally in liquid phase at room temperature and at typical operating temperatures of about 40° C. with a warm engine.
Previous attempts to improve part-to-part balancing in injector performance included precision injector calibration on flow rigs during manufacturing. However, as the injectors wear, parts change shape due to chemical swelling or uneven accumulation of contaminants, and the precision calibration can be greatly compromised. Fuel injector actuation issues can be mitigated (to a limited degree) by use of very strong magnetic opening forces, which can help to partially overcome resistance to motion or “stickiness” at the plunger/tube and valve seal/seat interfaces. However, stronger magnetic forces typically require higher peak coil current in the fuel injector actuator, which increases electrical energy consumption and reduces overall engine efficiency. In addition, using a coalescing filter upstream of fuel injectors reduces the amount of oil, water and dirt getting into the injectors. Contaminants can be in the gaseous fuel for a variety of reasons, such as oil from compressors that are employed to pressurize the gaseous fuel. Unfortunately, the necessary servicing of filters in the field cannot be guaranteed and the use of filters to reduce contaminants from reaching the injectors (and improving injector performance as a result) has had limited success. During cold start, engines that can be fuelled with gasoline and/or compressed natural gas (CNG) can avoid the “stickiness” of the gaseous fuel injectors by temporarily starting and running on gasoline to allow the engine to warm-up and reduce viscosity of the contaminants, and then switch to CNG after the engine has warmed up. These approaches do not directly deal with the root issue which is open-loop variability with injector age and low temperature (cold start) and low voltage (battery voltage) fuel injector operation.
The state of the art is lacking in techniques for improving injection accuracy for gaseous fuel injectors. The present apparatus and method provides a technique for operating a gaseous fuel injector in internal combustion engines.